I've been on a lot of Livestreams, and they're great. I noticed the people hosting the livestream talking. I've asked them about this and they simply say, "I use a headset." Well I recently got a headset for gaming purposes. I would like to be able to have my voice be heard during the Livestreams. Is there anyway to enable this feature? If so, I'd love if you'd share it. Thanks~

Livestream for Producers and Procaster features a built-in audio mixer, used to add and mix different audio sources simultaneously. You'll generally see an option for the headset in the audio mixer (If you're not live, it's accessible in the software's preferences --> audio tab)

What they were likely doing in the video is using the headset's audio source in addition to the Speaker's audio.